This invention relates generally to gas turbine engines and, more particularly, to a method and apparatus for minimizing rotor/shroud clearance during both steady-state and transient operation.
As turbine engines continue to become more reliable and efficient by changes in methods, designs and materials, losses which occur from excessive clearances between relatively rotating parts become more important in the many design considerations. In many turbine engine applications, there is a requirement to operate at variable steady-state speeds and to transit between these speeds as desired in the regular course of operation. For example, in a jet engine of the type used to power aircraft, it is necessary that the operator be able to transit to a desired speed whenever he chooses. The resulting temperature and rotor speed changes bring about attendant relative growth between the rotor and the surrounding shroud and, in order to maintain the desired efficiency, this relative growth must be accommodated. The primary concern is in maintaining a minimum clearnace between the stator and rotor while preventing any interference therebetween which would cause rubbing and resultant increase in radial clearance during subsequent operation. When considering the transient operating requirements as mentioned hereinabove, the relative mechanical and thermal growth patterns between the rotor and the shroud prevent a very difficult problem.
One of the primary reasons for wanting to maintain close clearance control during transient operating conditions as well as steady-state conditions is that of minimizing temperature overshoot in the engine. That is, during transient operation at a given thrust level, the turbine inlet temperature of the engine will exceed that for the same thrust level at a steady-state, stabilized condition, and the difference is known as the temperature overshoot. Higher temperature overshoots impose higher thermal gradients to the hot flow path parts and tend to reduce life in these parts. It will be understood that as turbine/shroud clearances are increased, and thus turbine efficiencies are decreased, it is necessary to operate at higher temperatures in order to meet designated thrust levels. Thus, greater clearances necessarily mean higher temperature overshoots and, conversely, temperature overshoots can be reduced by closely controlling these clearances.
Other reasons for close clearance control during transient and steady-state operation include the desirability of increasing stall margin and decreasing acceleration time.
Various schemes have been devised to variably position the stationary shroud in response to engine operating parameters in order to reduce rotor/shroud clearance. One such scheme is that of the thermal actuated valve as described in U.S. Pat. No. 3,966,354, issued on June 29, 1976 and assigned to the assignee of the present invention. In that apparatus, a valve operates in response to the temperature of the cooling air and, to the extent that the cooling air temperature is dependent on the speed of the engine, the transient condition is considered. However, such a system tends to be relatively slow in responding and relatively inaccurate in trying to match relative growth during transient operation.
Probably the primary reason that a cooling air system operating only on a speed responsive schedule is inadequate is that such a system is not capable of taking into account the thermal heating and cooling time constants of the rotor for all possible sequences of transitional operation. That is, present systems are only capable of matching thermal time constants of the rotor when the sequence of transient condition operation is known. This, of course, is not acceptable since the particular operating mode and sequence of operation is going to depend on the mission at hand.
It is therefore an object of the present invention to provide an efficient turbine engine which is capable of transiting between various speeds while maintaining a minimum clearance between its rotor and shroud.
Another object of the present invention is the provision in a turbine engine for responsively modulating the position of a rotor shroud in response to variable steady-state and transient operating conditions.
Yet another object of the present invention is the provision for reducing temperature overshoot during transient operation.
Still another object of the present invention is the provision in a gas turbine engine for increasing stall margin and decreasing acceleration time.
Yet another object of the present invention is the provision in a clearance control system for selectively varying the shroud position in response to the thermal time constants of the rotor.
Still another object of the present invention is the provision for a rotor/shroud clearance control system which is responsive and effective over a wide range of steady-state and transient operating conditions.
Yet another object of the present invention is the provision for a rotor/shroud clearance control system which is economical to manufacture and relatively simple in operation.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.
Briefly, in accordance with one aspect of the invention, a cooling air system is made more responsive and versatile by the use of two sources of air at different temperatures and providing for the use of those sources individually or together so that a total of four different cooling modes or temperatures may be used. Selection of the cooling mode or modes is determined automatically in response to the particular speed range in which the engine is operating and of the time it is operating within that range.
By another aspect of the invention, cooling air is scheduled to flow to the shroud support structure at a particular temperature sequence which corresponds to the thermal time constants of the rotor. In this way, the system operates in response to engine speed and time of operation of modulate the temperature of the cooling air flow in such a way as to obtain the optimum rotor/shroud clearance control during both transient and steady-state operations.
By yet another aspect of the invention, the speed of the rotor is sensed and, when the speed reaches a predetermined level, a timer is started and cooling air is sent to the support structure in accordance with a predetermined mixture schedule. This schedule is closely matched to the thermal time constants of the rotor and may be overridden by the rotor's attaining another predetermined speed wherein another cooling air mode of operation will be put into effect.
By still another aspect of the invention, cooling air temperatures are selectively varied by mixing air which is bled off from the fifth and the ninth stages of the compressor. Four different increasing levels of cooling air temperature can then be obtained by having either no air, fifth stage air only, a mixture of fifth and ninth stage air, or ninth stage air only. The particular sequence is determined by the speed of the rotor and the particular temperature is determined by the time of operation within that speed range.
In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.